Oct. 15, 1919 Rate of A hsorption of Soil Constituents 69 



seventh and ninth weeks, and in spite of a continued low nitrate concen- 

 tration we observe no further loss of nitrogen from the plant. The small 

 additional increment of nitrogen subsequently absorbed, above that 

 present at nine weeks, may be accounted for by concurrent nitrification 

 during the 6-week period between the ninth and fifteenth weeks. The 

 fact that the uncropped soil shows a loss of nitrate at this time is no 

 evidence against such an assumption, inasmuch as nitrification may have 

 been more intense in the cropped soil of low nitrate concentration. 



Potassium. — (Fig. 10.) As with nitrate, we observe here a lowered 

 concentration of the soil extract coincident with a high rate of absorption 

 by the plant and shortly afterward followed by a loss of the element from 

 the plant. A little later potassium is again taken up by the plant at a 

 period when the water-extractable potassium of the soil is increasing. If 

 a loss of water-extractable potassium indicates a lowered concentration 

 of the soil solution, some disturbance of the equilibrium between the 

 latter and the cell sap must occur. Increased concentrations of potas- 

 sium compounds with diosmotic properties in the cell sap would tend 

 still further to change the previously existing equilibria and, if sufficiently 

 great, account for a movement of potassium from the plant to the soil. 



CAI.CIUM AND MAGNESIUM. — (Figs. II, 12.) The Small magnitudes in- 

 volved in the fluctuations of these elements within the plant vitiate 

 any definite conclusions therefrom. It is interesting to note, however, 

 that there is a distinct loss of calcium and an apparent loss of magnesium 

 between the ninth and eleventh weeks, the period immediately following a 

 lowered concentration of the corresponding ions in the water extracts. 



Phosphorus. — (Fig. 13.) The water-soluble phosphorus of the soil 

 seldom shows the considerable fluctuations observed in other elements 

 concurrently with changes in the depression of the freezing point or with 

 changes of soil conditions suggestive of corresponding changes in the soil 

 solution. We can, therefore, hardly expect that the water-extractable 

 phosphorus will shed much light on the mechanism of the process 

 by which plants gain or lose very small increments of this constituent. 

 We may, however, point out^that the data have the minor merit of not 

 being inconsistent with the suggestion made in connection with other 

 constituents, in that phosphorus is the only element which appears to 

 increase between the ninth and eleventh weeks and the only one in which 

 the corresponding ion of the water extract does not decrease materially 

 just before this period. 



CONCLUSIONS 



The absorption of certain soil constituents by barley is characterized 

 by three distinct phases, coextensive with the more important stages of 

 vegetative development. The first of these covers a period of progres- 

 sively increasing rate of absorption, ending about the time the heads begin 

 to form. At this time the absolute amounts of potassium and nitrogen 



